This invention relates generally to the field of hand-held data collection and analyzer systems and more particularly, relates to an improved signal sampling process for such systems.
Hand-held data collection and analyzing systems are well known for collecting a variety of predictive maintenance data in numerous applications. Often, such collection and analyzer systems are used to collect data from rotating shafts representing vibrations created as a function of the shaft rotation. Such vibration data is analyzed to diagnose potential problems with components associated with the vibrating shaft. Such diagnostic information is used for predictive maintenance purposes and permits lesser problems to be detected and predicted prior to their maturing into more serious and more expensive problems to remedy.
One such hand-held data collector and analyzer system is described in U.S. Pat. No. 5,633,811, which, referring to FIG. 3 of the ""811 patent, has an analog front end in which a vibration signal on line 19 is filtered and otherwise processed by analog components 40-50. The processed analog signal on line 22 is converted to a corresponding digital signal by an analog/digital converter 52. Thereafter, the digital signal is conditioned with digital filters 54, 56 prior to being analyzed by the data processor 28. The analog/digital converter 52 samples the input signal at 1 of 8 selectable sampling frequencies, the sampling frequency being chosen as a function of the speed of the rotating shaft (not shown) providing the vibration signal on line 19.
Rotating bodies produce periodic vibrations and acoustic signals related to fundamental rotational speed and its harmonics which are integer multiples of the rotational speed. In the above system as well as other data collection systems, upon a frequency of interest being chosen by the user, a fast fourier transform (FFT) is performed on the digitized vibration signal. The FFT transforms the revolution domain data into a frequency spectrum. It is generally assumed that when the vibration data used in performing the FFT is being collected, the machine or shaft speed is constant. A shaft rotating at a constant speed will provide a locus of output vibration samples forming a constant frequency time waveform similar to that illustrated in FIG. 8. A vibration signal sampled therefrom has a fixed relationship to the rotational speed of the shaft, and provides an FFT display as illustrated in FIG. 9 in which the frequency spectrum is concentrated in one bin.
Often, during normal operation, rotating shafts experience a change in speed. There are many reasons for such a change in speed, for example, a change in shaft load or, in a winding process, the material being wound is preferably moving at a constant linear speed. If the shaft speed changes, a time waveform is produced as illustrated in FIG. 10 in which a locus of output vibration samples are provided over equal increments of time as a function of the input sample rate. However, during the vibration signal sampling interval, the periodicity of the vibration signal from the rotating shaft shifts slightly in phase with respect to the fixed sampling, and thus, a different number of samples is provided for each different period of the input frequency spectrum. This causes the FFT record to smear, that is, as illustrated in FIG. 11, the frequency spectrum spreads over a frequency band corresponding to the speed change.
Such variations in shaft speed may be detected and compensated for in large laboratory equipment for performing vibration analysis. However, portable hand-held instrumentation is required to be as light, small and low powered as possible, and there are strict limitations on how such compensation can be implemented.
Therefore, there is a need to provide a portable hand-held instrument for collecting and analyzing vibration data that does not have the above-described disadvantage.
The present invention provides a simple, compact, inexpensive and reliable portable hand-held instrument for collecting vibration data that is independent of changes in speed of a rotating shaft inducing the vibration. The invention is especially useful in performing an order tracking frequency domain analysis, for example, a FFT analysis on collected vibration data induced by a shaft rotating at different speeds.
According to the principles of the present invention and in accordance with one embodiment, a hand-held data unit for collecting vibration data induced by a rotating shaft has a first input adapted to receive an analog vibration signal representing vibrations related to the rotating shaft and a second input adapted to receive a reference signal representing a current speed of the rotating shaft. A digital processor first converts the analog vibration signal into corresponding sets of digital vibration samples. Thereafter, the digital processor provides in response to the sets of the digital vibration samples, corresponding sets of output vibration samples in which samples within each set of output vibration samples are provided at a rate that varies as a function of variations in the speed of the rotating shaft. The digital processor then collects a time waveform record in response to a plurality of the output vibration samples, wherein the frequency of the output vibration samples in the plurality of output vibration samples collected in the time waveform record varies as a function of changes in the speed of the rotating shaft. Thus, the invention has the advantage of permitting a nonsmearing, high quality display of an order tracking frequency domain analysis, for example, a FFT analysis, using output vibration samples that are collected from a variable speed rotating shaft.
In one aspect of the system, the analog vibration signal is converted into sets of digital vibration signals over fixed intervals of time. Further, sets of output vibration samples are provided over the fixed intervals of time wherein the rate of the samples in each set varies as a function of variations in the speed of the rotating shaft.
In another aspect of the invention, the rotating shaft can be represented by a periodic mechanical system and output vibration samples are collected in a time waveform record in which the frequency of the output vibration samples varies as a function of variations in the frequency of the periodic mechanical system.
In another embodiment of the invention, a method of processing analyzing vibration data induced by a rotating shaft using a hand-held data collection and analyzing unit has a first step of collecting and converting input analog vibration signals into corresponding digital vibration samples over fixed intervals of time. Next the method provides in response to the digital vibration samples, corresponding sets of output vibration samples in which the output vibration samples in each set having sample rates that vary as a function of variations in the current angular speed of the rotating shaft. Thereafter, the method collects a time waveform record in response to at least one of the sets of the output vibration samples wherein the frequency of the output vibration samples in the one set of output vibration samples varies as a function of variations in the speed of the rotating shaft.
These and other objects and advantages of the present invention will become more readily apparent during the following detailed description taken in conjunction with the drawings herein.